A type of hybrid vehicle has been known in which an internal combustion engine and a motor serving as drive sources, and drive-by-motor operation is carried out by stopping the engine and actuating only the motor as the drive source. Through the drive-by-motor operation, intermittent control is performed to improve the fuel efficiency of the engine. In accordance with the intermittent control, the drive-by-motor operation is carried out only when a prescribed executing condition is satisfied. Specifically, the drive-by-motor operation is carried out when the engine temperature is greater than or equal to a permitting temperature (which is, for example, an engine warm-up completion temperature) and prohibited when the engine temperature is less than the permitting temperature.
The drive-by-motor operation in accordance with the intermittent control allows a hybrid vehicle to reduce the fuel consumption of the engine. Accordingly, after the system start-up is initiated in the hybrid vehicle, the fuel efficiency of the engine is further improved as the time becomes longer during which the intermittent control is maintained. The system start-up in the hybrid vehicle herein refers to the state where every device necessary for the vehicle to travel receives power so that the engine can be started and the drive-by-motor can be executed. To initiate the system start-up in the hybrid vehicle when the engine is cold, the engine is started simultaneously with initiation of the system start-up so that the temperature of the engine can rise. Accordingly, to improve the fuel efficiency of the engine, it is preferable to increase the engine temperature to the permitting temperature or higher as early as possible to execute the intermittent control at an early stage after initiation of the system start-up in the hybrid vehicle with the engine cold.
To achieve the goal, the energy generated through operation of the engine at the time when the hybrid vehicle is in operation may be stored in a storage device. If the engine is cold when subsequent system start-up is initiated in the hybrid vehicle, the engine may be heated by a heating device using the energy stored in the storage device. This raises the engine temperature to the permitting temperature or higher at an early stage after initiation of the system start-up, thus executing the intermittent control. As a result, the intermittent control allows the fuel efficiency of the engine to be improved.
As described in Patent Document 1, heat exchange fluid (coolant) circulates in a fluid circuit and passes through an engine to cause heat exchange to cool the engine. Specifically, when the engine is in operation and the coolant is heated, the heated coolant is sent to and stored in a heat storage container via an inlet passage. The engine is thus heated by the thermal energy stored in the heat storage container in the form of the heated coolant. Specifically, the heated coolant in the heat storage container is sent from an outlet passage to the fluid circuit and supplied to the engine through the fluid circuit. This heats the engine and allows the engine temperature to be raised to the permitting temperature or higher. Accordingly, in this case, the heat storage container functions as a storage device for storing thermal energy. The outlet passage and the fluid circuit each function as a heating device for heating the engine using the thermal energy.
In a hybrid vehicle, a motor, together with a generator and an inverter, is mounted in a transaxle. The temperature of the transaxle also influences fuel efficiency of the engine to a great extent. In other words, when the temperature of the transaxle including the motor or the like is low, oil viscosity in a gear portion of the transaxle may become high, thus increasing resistance of the transaxle (the gear portion) to operation of the engine. Also, drive efficiency of the motor may decrease and thus increases resistance of the motor to the operation of the engine. As a result, the fuel efficiency of the engine is deteriorated. To solve this problem, if the engine and the transaxle are cold when the system start-up is initiated in the hybrid vehicle, not only the engine but also the transaxle may be heated by the energy stored in the storage device so as to improve the fuel efficiency of the engine.
However, when the energy stored in the storage device is used to heat the transaxle in addition to the engine, it is likely that the engine temperature cannot reach the permitting temperature even after all of the energy is consumed. In this case, the intermittent control cannot be executed, thus preventing effective improvement of the fuel efficiency of the engine from being achieved through the intermittent control. This problem is highly likely to occur particularly in the case of Patent Document 1, in which the thermal energy generated through the engine operation is stored in the storage device in the form of the heated coolant and the heated coolant is supplied to the engine and the transaxle to heat the engine and the transaxle using the aforementioned thermal energy. Specifically, the problem is brought about by the fact that, in recent hybrid vehicles, engines are reduced in size and exhibit high thermal efficiency, resulting in less heat generation by the engines and a lowered temperature of coolant stored in storage devices.
This problem is generally common in hybrid vehicles that have a battery to store electric energy produced by a generator at the time when an engine is in operation and heats the engine and a transaxle by means of an electrothermal heater using the stored electric energy in subsequent system start-up. In this case, the battery functions as a storage device and the electrothermal heater functions as a heating device.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2007-55299 (Paragraphs [0002] and [0003] and FIG. 1)